Catalytic Asymmetric syntheses Using Vitamin B_<12> as a Chiral Catalyst

维生素B_<12>与具有吸电子取代基的环丙烷如乙酰基、甲氧羰基和氰基反应生成3取代丙基钴配合物。与前手性1-乙酰基-1-丙基环丙烷的烷基化反应导致烷基配体中碳3的不对称诱导（ee 24-33%）。对烷基化产物的^1H NMR谱分析表明：(1)3,3-二乙酰丙基-和3,3-二（甲氧羰基）丙基-钴配合物中的两个前手性甲基由于手性B_<12>的存在而呈现非对映异构，且在光谱上不等效；(2)3-乙酰基-3-烷基丙基-和3-乙酰基-3-（甲氧羰基）丙基-钴配合物中的对映体甲基在碳3上具有不对称中心，也呈现非对映异构以相似的方式在光谱上可区分的这样得到的Co- c衍生物经过光化学均解得到Co（II）和烷基自由基，从异丙醇中提取一个氢原子。因此，B_<12>在15% aq. NH_4 cl -异丙醇（1:1）含Zn粉尘中催化环丙烷在光化学条件下的裂解还原。当1-苯基-1-甲氧基羰基环丙烷作为底物时，观察到大量的不对称还原。在迈克尔烯烃（丙烯酸甲酯、甲基丙烯酸酯或克罗东酸酯）存在下，B_<12>也催化环丙烷衍生的C-3单元的迈克尔加成反应。其反应机理包括：(1)Co- c衍生物光化学均解生成Co（II）和烷基自由基；(2)自由基在Michael受体上的均解Michael加成；(3)异丙醇产生的羟基的氢萃取；(4)Co(II) Zn还原为Co(I); (5) Co(I)与环丙烷反应再生Co- c衍生物。

Vitamin B_<12> reacts with cyclopropanes having electron-withdrawing substituents such as acetyl, methoxycarbonyl, and cyano groups to give 3-substituted propyl-cobalt complexes. The alkylation with prochiral 1-acetyl-1-aklylcyclopropanes results in an asymmetric induction (ee 24-33%) at carbon 3 in the resulting alkyl ligands. Examination of the ^1H NMR spectra of the alkylation products indicates that (1) two prochiral methyl groups in 3,3-diacetylpropyl- and 3,3-bis(methoxycarbonyl)propyl-cobalt complexes are rendered diastereotopic by the presence of the chiral B_<12> and are observed to be spectroscopically nonequivalent and (2) enantiomeric methyl groups in 3-acetyl-3-alkylpropyl- and 3-acetyl- 3-(methoxycarbonyl)propyl-cobalt complexes having an asymmetric center at carbon 3 are also rendered diastereotopic and spectroscopically distinguishable in a similar manner.The Co-C derivatives thus obtained undergo photochemical homolysis to give Co(II) and alkyl radicals which abstract a hyrogen atom from isopropanol. Thus, B_<12> catalyzes the ring-cleavage reduction of cyclopropanes in 15% aq. NH_4 Cl-isopropanol (1:1) containg Zn dust under photochemical conditions. When 1-phenyl-1-methoxycarbonylcyclopropane is used as substrate substantial asymmetric reduction is observed. In the presence of Michael olefins (methyl acrylate, methacrylate, or crotonate), B_<12> also catalyzes the Michael addition of cyclopropane-derived C-3 unit. The mechanism involves (1) photochemical homolysis of a Co-C derivative to give Co(II) and an alkyl radical, (2) homolytic Michael addition of the radical to a Michael acceptor, (3) hydrogen abstraction of the resulting ardical from isopropanol, (4) Zn reduction of Co(II) to Co(I), and (5) reaction of Co(I) with the cyclopropane to regenerate Co-C derivative.